Hardibacker install around bath tub

I am installing 1/2" hardibacker cement board (Hardiebacker 500) around bath tub. I have read somewhere on other forums about leaving 1/8" gap between sheets to compensate for an expansion. However, install instructions posted on James Hardie's website only recommends leaving 1/8" gap between sheet edges for countertop and subfloor installs, and instructs not to force edges together. Nothing about leaving gaps for wall installs.

I wonder if the 1/8" gap between boards is really necessary. The cement board appears to be structurally solid. Please share your experiences.

A little space won't hurt. Further down in the instructions there's a section on using alkali resistant mesh tape on the joints. This is important, and makes the assembly more monolithic like a mudded wall. A slight gap gives a place to smush the thinset in.

You should also be installing a vapor barrier behind the Hardie, lapped over the tiling flange of the tub and the Hardie should terminate about 1/8-1/4" above the edge of the tub deck. Since most of the flanges are thick, you can either notch the studs to make that flange flush so the Hardie can be run down over it, or stop it slightly above the flange. The bottom row of tile can overlap that area as long as at least 1/2 of the tile is supported on the CBU, which limits the size of the tile that can be used (i.e., you can't use really small tile and a high flange unless you notch the studs to allow the cbu to cover it).

Silica dust is a known carcinogin in the same class as asbestos...neither are a problem if you don't inhale them. If you cut Hardibacker with a saw verses snapping it, it produces HUGE clouds of the stuff - if you value your health, don't do it without understanding the ramifications and consequences. It would be hard to notch the back of Hardie without power tools, thus, the warning. One of the real cement boards might be able to be notched, but it is often then so weak, I'd not recommend it. I'd either notch the studs, or leave it above the flange and make sure I used a tile size that could handle it.

Once the Hardie is up and the seams taped with thin set and mesh give the entire surround a couple days to cure out. Then a couple of coats of liquid membrane would be advised and you have a bullet proof installation.

Since you have gone to the trouble to leave an expansion gap don't fill it with thin set and waterproofing. I like to tape a sliver of sill gasket before these steps and leave this in place until after tiling. Once the tile and grout are done you can slice the sill gasket back and fill with silicone.

Good Afternoon,
New to the forum and have a question regarding the last statement John made. I have been doing lots of research before completing my tub/shower install. I have decided on the Hardie backer with a liquid water proofer (Redguard). I thought I read if using a liquid waterproofer, you should not install a vapor barrier behind the backer board. Can you clarify this?

Neither tile nor Hardie are totally waterproof, but are pretty much impervious to moisture damage. So, it's not great to encapsulate the Hardie with a sealer on both sides. In case it does get wet for any reason, it would take forever to dry out - it needs a path for any moisture to be released.